FIG. 19 is a view illustrating the construction and operation of a conventional industrial robot, which is shown in, for instance, Japanese Patent Publication No. SHO 63-36914. Referring to the Figure, designated at 1' is a stationary link, at 2' a rotational link, at 3' a rotational joint, at 4' a tension coil spring, at 5' a movable support end, at 6' a stationary support end, at 7a' and 7b' directions (g) of gravitational acceleration, and at 8'a base mounting part (such as a floor or a ceiling).
With the above construction of the conventional robot, the distance L between the movable and stationary support ends 5' and 6' varies in dependence on the posture of the rotational link 2'. Therefore, the gravitational torque exerted to an actuator (not shown) for driving the rotational joint 3' is compensated for with the restoring force of the tension coil spring 4' connected between the movable and stationary support ends 5' and 6'.
With this gravitational torque compensation, it is possible to reduce the capacity of the actuator, and the gravitational balancer thus may be used for the purposes of reducing the shape, weight and cost.
The operation will be described by assuming that a counterclockwise torque about the rotational joint 3' is a positive torque. Denoting the free length of the tension coil spring 4' by L.sub.N, the spring constant thereof by k and the initial tension by F, the compensation torque provided by the spring 4' by Tc can be obtained using the following equations 1. EQU Tc={k(L-L.sub.N)+F}.multidot.1.multidot.r.multidot.sin .theta./L,
and EQU L.sup.2 =1.sup.2 +r.sup.2 -2.multidot.1.multidot.r.multidot.cos .theta.(Equations 1)
wherein,
L: Length of the spring 4' (a distance between the movable support edge 5' and the fixed support edge 6') PA1 l: A distance between the movable support edge 5' and the center of the rotary joint 3' when the rotary line 2' is in the vertical posture PA1 r: A distance between the center of the rotary joint 3' and the fixed support edge 6' PA1 .theta.: An angle of the rotary link 2' PA1 Tc: Compensation torque by the spring 4' PA1 k: Spring constant of the spring 4' PA1 F: Initial tension of the spring 4'
FIG. 20 shows examples of the compensation torque Tc exerted to the spring 4'. The Figure also shows examples of the gravitational torque in case when the gravitational balancer is installed on a floor, in which case the gravitational acceleration is exerted in a direction 7a' (shown in FIG. 19). In the case of installation on a floor, the gravitational torque or gravitational force is considerably reduced by the compensation torque provided by the spring 4', and the sum of the gravitational torque exerted to the actuator for driving the rotational joint 3' and the compensation torque Tc provided by the spring 4' is sufficiently less than the gravitational torque in the case, in which the spring 4' is not provided. Thus, the gravitational balancer provides for effective action.
FIG. 21 shows a conventional industrial robot disclosed in Japanese Patent Publication No. SHO 63-36914 described above, and referring to this figure, designated at 100 is a stationary rest, at 20 is a rotational base rotationally supported on the stationary rest 100, at 300 is a first arm rotationally supported on the rotational base 200, at 350 a second arm rotationally supported on the first arm, at 330 a movable support end formed in the neighborhood of a center for rotation of the second arm on the first arm 300, at 420 a stationary support end formed in the neighborhood of a center for rotation of the first arm 300 on the rotational base 200, at 440 a spring with one end thereof rotationally supported on the movable support end 330 and another end thereof rotationally supported on the stationary support end 420, and at 310 a second revolution driving motor to drive the first arm 300 into revolution.
The operation will now be described. When the first arm 300 is in an inclined posture against the vertical direction, the second revolution driving motor is required to generate a torque for the first arm 300 and the second arm 350 to maintain their posture resisting a gravitational torque causing the arms to drop naturally. For this reason, when the first arm is inclined, also the movable support end 330 is inclined, and the spring 440 extends as compared to in its vertical posture to generate a tensile force. The tensile force works in a direction offsetting the gravitational torque, thus the load to the second revolution driving motor is alleviated.
Among other reference technical literatures relating to the present invention, there are "Gravitational Balancer" disclosed in Japanese Patent Laid-Open No. SHO 55-35735, "Spring Assembly for Balancing" disclosed in Japanese Patent Application Laid-Open No. SHO 63-221991 and "Gravitational Balancer" disclosed in Japanese Patent Application Laid-Open No. HEI 4-19092.
The above conventional industrial robot, however, has a problem. That is, although it provides its function when it is installed on a floor, it does not provide the function in other installation postures (for instance when it is hung from a ceiling).
This will now be described with reference to FIGS. 19 and 20. In the case of hanging from a ceiling, in which case the gravitational (g) acceleration is exerted in a direction 7b' shown in FIG. 19, the sign of the gravitational torque is changed from that in the case of installation on a floor shown in FIG. 20. In this case, the absolute value of the sum of the gravitational torque exerted to the actuator for driving the rotational joint 3' and the compensation torque Tc provided by the spring 4' is considerably greater than the absolute value of the gravitational torque in the case in which the spring 4' is not provided. Obviously, therefore, the gravitational balancer in this case does not provide a desired function.
With the above construction of the conventional industrial robot, an effect to alleviate a load to a driving motor when the arm is in an inclined posture is provided, but a spring must be disposed in the outer side from the first arm, so that a position to support the spring is limited and optimization of the spring characteristics can not be achieved by adjusting its mounting position. In addition, as excellent appearance is required for industrial robots in recent years, such problems as exposition of a spring to the outside of an industrial robot detracts from the appearance of the industrial robot. Finally, reliability is reduced due to such causes as scattering of broken pieces of the spring, when broken, or biting of foreign materials.